This topic has been greatly debated over the decades.
Before I tell you my opinion on this, let me first explain...
What Is Tuition?
Google defines it as "teaching or instruction, especially of individual pupils or small groups."
Tuition is basically like extra class but not at school. It's meant for students to improve in subjects they're weak in but nowadays even those doing exceptionally well go for multiple tuition classses. In my country, Malaysia, almost all students from national(government) schools go for tuition while in private & international schools almost none do. Generally, there are four classes a month with each class lasting about 2 hours. On average, tuition costs about RM60(16.63 USD) a month.
Why Is It So Popular?
The main reason is probably because the belief is that tuition = good exam results = schoarships especially in Malaysia where scholarships are literally given by the number of As you get. I believe it is also because we Asians tend be fiercely competitive(the 3 Ks; kiasu, kiasi, kiabo) and result oriented especially when it comes to academics. Asian parents place a lot of importance on excelling academically which is not a bad thing considering the fact that academic excellence is needed for basically everything but sometimes they may go overboard. It's the thought that counts, right?